Since that experience, I have traced this limit of southern boulders for thousands of miles across the continent, according to the delineation which may be seen in the [map in a later chapter]. If necessary, I could indicate hundreds of places where the proof of glacial transportation is almost as clear as that on the Pocono plateau in Pennsylvania. One of the most interesting of these is on the hills in Kentucky, about twelve miles south of the Ohio River, at Cincinnati, where I discovered boulders of a conglomerate containing many pebbles of red jasper, which can be identified as from a limited formation cropping out in Canada, to the north of Lake Huron, six hundred or seven hundred miles distant. That this was transported by glacial ice, and not by floating ice, is evident from the fact that here, too, there was no barrier to the south, requiring deposits to cease at that point, and from the further fact that boulders of this material are found in increasing frequency all the way from Kentucky to the parent ledges in Canada. With reference to these boulders, as with reference to those found on the summit of Mount Washington, we can reason, also, that any northerly subsidence permitting a body of water to occupy the space between Kentucky and Lake Superior, and deep enough to facilitate the movement across it of floating ice, would render it impossible for the ice to have loaded itself with them.
Fig. 25.—Conglomerate boulder found in Boone County, Kentucky. (See text.)
The same line of reasoning is conclusive respecting the innumerable boulders which cover the northern portion of Ohio, where I have my residence. The whole State of Ohio, and indeed almost the entire Mississippi basin between the Appalachian and the Rocky Mountains, is completely covered, and to a great depth, with stratified rocks which have been but slightly disturbed in the elevation of the continent; yet, down to an irregular border-line running east and west, granitic boulders everywhere occur in great numbers. In the locality spoken of in northern Ohio the elevation of the country is from two hundred to five hundred feet above the level of Lake Erie. The nearest outcrops of granitic rock occur about four hundred miles to the north, in Canada. After the meeting of the American Association for the Advancement of Science in Toronto in the summer of 1889, I had the privilege of joining a company of geologists in an excursion, conducted by members of the Canadian Survey, to visit the region beyond Lake Nipissing, north of Lake Huron, where the ancient Laurentian and Huronian rocks are most typically developed. I took advantage of the trip to collect specimens of a great variety of the granites and gneisses and metamorphic schists and trap-rock of the region. On bringing them home I turned them over to the professor of geology, who at once set his class at work to see if they could match my fragments from Canada with corresponding fragments from the boulders of the vicinity. To the great gratification, both of the pupils and myself, they were able to do so in almost every case; and so they might have done in any county or township to the south until reaching the limit of glacier action which I had previously mapped. Here, at Oberlin, on the north side of the water-shed, it is possible to imagine that we are on the southern border of an ancient lake upon whose bosom floating ice had brought these objects from their distant home in Canada. But this theory would not apply to the portion of the State which is south of the water-shed and which slopes rapidly towards the Gulf of Mexico. Yet the distribution of boulders is practically uniform over the glaciated area on both sides of the water-shed, constituting thus an indisputable proof of the glacial theory.
4th. As the significance of the gravel terraces which mark the lines of outward drainage from the glaciated area cannot well be indicated in a single paragraph, the reader is referred for further information upon this point to the general statements respecting them throughout the next chapter.
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